Re: [FFmpeg-devel] [PATCH v2] avcodec/startcode: Avoid unaligned accesses

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Anton Khirnov:
> Quoting Andreas Rheinhardt (2022-10-12 00:20:23)
>> Up until now, ff_startcode_find_candidate_c() simply casts
>> an uint8_t* to uint64_t*/uint32_t* to read 64/32 bits at a time
>> in case HAVE_FAST_UNALIGNED is true. Yet this ignores the
>> alignment requirement of these types as well as effective type
>> rules of the C standard. This commit therefore replaces these
>> direct accesses with AV_RN64/32; this also improves
>> readability.
>>
>> UBSan reported these unaligned accesses which happened in 233
>> FATE-tests involving H.264 and VC-1 (this has also been reported
>> in tickets #8138 and #8485); these tests are fixed by this commit.
>>
>> The output of GCC with -O3 is unchanged for aarch64, alpha, arm,
>> loongarch, ppc and x64. There was only a slight difference for mips.
>>
>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
>> ---
>> This is v2 of https://patchwork.ffmpeg.org/project/ffmpeg/patch/20200122145210.6898-1-andreas.rheinhardt@gmail.com/
>>
>> Here is the mips code before this change:
>>
>> startcode_old_O3.o:     file format elf64-tradlittlemips
>>
>>
>> Disassembly of section .text:
>>
>> 0000000000000000 <ff_startcode_find_candidate_c>:
>>    0:	18a00029 	blez	a1,a8 <ff_startcode_find_candidate_c+0xa8>
>>    4:	3c08ff7f 	lui	a4,0xff7f
>>    8:	3c07ff01 	lui	a3,0xff01
>>    c:	65087f7f 	daddiu	a4,a4,32639
>>   10:	64e70101 	daddiu	a3,a3,257
>>   14:	00084438 	dsll	a4,a4,0x10
>>   18:	00073c38 	dsll	a3,a3,0x10
>>   1c:	65087f7f 	daddiu	a4,a4,32639
>>   20:	64e70101 	daddiu	a3,a3,257
>>   24:	00084478 	dsll	a4,a4,0x11
>>   28:	00073df8 	dsll	a3,a3,0x17
>>   2c:	00803025 	move	a2,a0
>>   30:	00001025 	move	v0,zero
>>   34:	3508feff 	ori	a4,a4,0xfeff
>>   38:	10000005 	b	50 <ff_startcode_find_candidate_c+0x50>
>>   3c:	34e78080 	ori	a3,a3,0x8080
>>   40:	24420008 	addiu	v0,v0,8
>>   44:	0045182a 	slt	v1,v0,a1
>>   48:	10600015 	beqz	v1,a0 <ff_startcode_find_candidate_c+0xa0>
>>   4c:	00000000 	nop
>>   50:	dcc30000 	ld	v1,0(a2)
>>   54:	0068482d 	daddu	a5,v1,a4
>>   58:	44a30000 	dmtc1	v1,$f0
>>   5c:	44a90800 	dmtc1	a5,$f1
>>   60:	4be10002 	pandn	$f0,$f0,$f1
>>   64:	44230000 	dmfc1	v1,$f0
>>   68:	00671824 	and	v1,v1,a3
>>   6c:	1060fff4 	beqz	v1,40 <ff_startcode_find_candidate_c+0x40>
>>   70:	64c60008 	daddiu	a2,a2,8
>>   74:	0045182a 	slt	v1,v0,a1
>>   78:	10600009 	beqz	v1,a0 <ff_startcode_find_candidate_c+0xa0>
>>   7c:	0082182d 	daddu	v1,a0,v0
>>   80:	10000005 	b	98 <ff_startcode_find_candidate_c+0x98>
>>   84:	90640000 	lbu	a0,0(v1)
>>   88:	24420001 	addiu	v0,v0,1
>>   8c:	10a20008 	beq	a1,v0,b0 <ff_startcode_find_candidate_c+0xb0>
>>   90:	00000000 	nop
>>   94:	90640000 	lbu	a0,0(v1)
>>   98:	1480fffb 	bnez	a0,88 <ff_startcode_find_candidate_c+0x88>
>>   9c:	64630001 	daddiu	v1,v1,1
>>   a0:	03e00008 	jr	ra
>>   a4:	00000000 	nop
>>   a8:	03e00008 	jr	ra
>>   ac:	00001025 	move	v0,zero
>>   b0:	03e00008 	jr	ra
>>   b4:	00a01025 	move	v0,a1
>> 	...
>>
>> And here after this change:
>>
>> startcode_new_O3.o:     file format elf64-tradlittlemips
>>
>>
>> Disassembly of section .text:
>>
>> 0000000000000000 <ff_startcode_find_candidate_c>:
>>    0:	18a0002b 	blez	a1,b0 <ff_startcode_find_candidate_c+0xb0>
>>    4:	3c08ff7f 	lui	a4,0xff7f
>>    8:	3c07ff01 	lui	a3,0xff01
>>    c:	65087f7f 	daddiu	a4,a4,32639
>>   10:	64e70101 	daddiu	a3,a3,257
>>   14:	00084438 	dsll	a4,a4,0x10
>>   18:	00073c38 	dsll	a3,a3,0x10
>>   1c:	65087f7f 	daddiu	a4,a4,32639
>>   20:	64e70101 	daddiu	a3,a3,257
>>   24:	00084478 	dsll	a4,a4,0x11
>>   28:	00073df8 	dsll	a3,a3,0x17
>>   2c:	00803025 	move	a2,a0
>>   30:	00001025 	move	v0,zero
>>   34:	3508feff 	ori	a4,a4,0xfeff
>>   38:	10000005 	b	50 <ff_startcode_find_candidate_c+0x50>
>>   3c:	34e78080 	ori	a3,a3,0x8080
>>   40:	24420008 	addiu	v0,v0,8
>>   44:	0045182a 	slt	v1,v0,a1
>>   48:	10600017 	beqz	v1,a8 <ff_startcode_find_candidate_c+0xa8>
>>   4c:	00000000 	nop
>>   50:	68c30007 	ldl	v1,7(a2)
>>   54:	6cc30000 	ldr	v1,0(a2)
>>   58:	0068482d 	daddu	a5,v1,a4
>>   5c:	44a30000 	dmtc1	v1,$f0
>>   60:	44a90800 	dmtc1	a5,$f1
>>   64:	4be10002 	pandn	$f0,$f0,$f1
>>   68:	44230000 	dmfc1	v1,$f0
>>   6c:	00671824 	and	v1,v1,a3
>>   70:	1060fff3 	beqz	v1,40 <ff_startcode_find_candidate_c+0x40>
>>   74:	64c60008 	daddiu	a2,a2,8
>>   78:	0045182a 	slt	v1,v0,a1
>>   7c:	1060000a 	beqz	v1,a8 <ff_startcode_find_candidate_c+0xa8>
>>   80:	0082182d 	daddu	v1,a0,v0
>>   84:	10000006 	b	a0 <ff_startcode_find_candidate_c+0xa0>
>>   88:	90640000 	lbu	a0,0(v1)
>>   8c:	00000000 	nop
>>   90:	24420001 	addiu	v0,v0,1
>>   94:	10a20008 	beq	a1,v0,b8 <ff_startcode_find_candidate_c+0xb8>
>>   98:	00000000 	nop
>>   9c:	90640000 	lbu	a0,0(v1)
>>   a0:	1480fffb 	bnez	a0,90 <ff_startcode_find_candidate_c+0x90>
>>   a4:	64630001 	daddiu	v1,v1,1
>>   a8:	03e00008 	jr	ra
>>   ac:	00000000 	nop
>>   b0:	03e00008 	jr	ra
>>   b4:	00001025 	move	v0,zero
>>   b8:	03e00008 	jr	ra
>>   bc:	00a01025 	move	v0,a1
>>
>> As one can see, the difference is that an ld has been replaced
>> by a pair of ldl and ldr. I don't know the performance implications
>> of this.
> 
> google tells me ld is an aligned load, so this change is then correct
> whatever its performance implications are.
> 

I was about to write the same after reading
https://www.cs.cmu.edu/afs/cs/academic/class/15740-f97/public/doc/mips-isa.pdf,
but then I found out that certain loongson processors allow unaligned
accesses (see
https://lists.gnu.org/archive/html/qemu-devel/2020-10/msg08563.html) and
configure only enables fast_unaligned for
loongson2e|loongson2f|loongson3*); if fast_unaligned would lead to
crashes on these processors, it would probably have been noticed long
ago. So the question of performance implications on such processors is
still open.
If no one bothers to test this in a reasonable time, I will treat this
as sign that no one is interested in the affected arches and apply this.

- Andreas

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